Reinforced lead screw with anti-backlash nut

ABSTRACT

An anti-backlash nut is assembled on a reinforced lead screw for bilateral motion lengthwise of the lead screw as the screw is rotated. A hollow, elongate reinforcing rail extends lengthwise of and surrounds the lead screw. A slot in the rail extends lengthwise of the central axis of the lead screw. The anti-backlash nut comprises two separate portions, each portion having a tongue which extends radially through the slot and has threads engageable with the threads of the lead screw for moving both nut portions in reciprocating motion lengthwise of the rail when the screw is rotated. Spring means are employed for forcing the nut portions apart to urge the flanges of their threads into forcible engagement with the threads of the lead screw to prevent backlash, while the anti-backlash nut translates laterally along the reinforcing rail.

RELATED APPLICATIONS

This application is a Continuation-in-Part of application Ser. No.09/336,905 filed Jun. 21, 1999 now U.S. Pat. No. 6,202,500 which is aContinuation-in-Part of Ser. No. 08/842,849 filed Apr. 17, 1997, nowU.S. Pat. No. 5,913,941, the contents of the which are incorporatedherein by reference in their entirety.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,566,345, issued Jan. 26, 1986 to the present inventorsand disclosed a carrier for mounting a tool for reciprocating motionalong a pair of spaced, parallel guide rails. The carrier itself isreciprocated bilaterally by a rotating lead screw extending lengthwisebetween and parallel to the guide rails. The lead screw engages aninternally threaded nut on the carrier. The nut may be an anti-backlashnut and will be so illustrated hereinafter. With the lead screw locatedbetween the rails, there are bearing supports extending laterally fromthe carrier, each mounting a bearing which slides on a guide rail muchin the manner of outrigger pontoons.

The rails serve a number of purposes. They create a low-frictionguideway for the carrier to ride on but, more importantly, they createan anti-rotation mechanism for the carriage. If there were no guiderails and the carrier were mounted on the lead screw only, the carrierprobably would not reciprocate. This is because the mass of the carrierwould create high frictional resistance between its threaded nut and thelead screw such that the carrier would rotate with the lead screw ratherthan traversing it. Secondly, again, if there were no guide rails and ifthe lead screw were oriented in a horizontal direction, the flexure ofthe lead screw, created by the weight of the carrier and the tool thatit mounts, could cause the carrier to bind on the lead screw and/orcause the tool that it mounts to disengage from its workpiece.

Yet another problem can exist. Tools mounted on the carrier are normallyoffset from the central axis of the lead screw. In addition, the toolsare mounted on posts in a cantilevered position relative to the carrier.Normal drag of the tool relative to its workpiece creates a torque whichis imparted through the tool post to the carrier and, hence, to thesupports riding on the rails which carry the bearings. This can causeunnecessary drag, monkey-biting, or can conceivably result in the leadscrew's driving motor to stall out.

Another shortcoming in certain applications where space is limited isthe size of the mechanism. Two guide rails spaced laterally of a leadscrew creates a drive mechanism which is spread out, generallyhorizontally, and, in some machine operations, there is not sufficientspace to do this. In addition, it is difficult to obtain initialalignment of the screw and two parallel rails.

It is to the solution of these problems that the present invention isdirected.

SUMMARY OF THE INVENTION

The invention resides in a reinforced lead screw assembly which includesa threaded lead screw rotatable about a central axis by a reversiblemotor. Surrounding the lead screw is a rigid, hollow, elongatereinforcing rail or tube which extends lengthwise end to end of thescrew. A two-part anti-backlash nut is movable along the reinforcingrail and is driven by the screw. There is a slot in the reinforcing railwhich extends lengthwise of the central axis of the lead screw.Projections or tongues on both portions of the nut extend in a radialdirection through the slot in the reinforcing rail and are engageablewith the walls of the slot to prevent rotation of the anti-backlash nutrelative to the rail. The tongues or projections mount threads which areengageable with the threads of the lead screw. Consequently, when thelead screw is rotated in either direction, the nut will translatelengthwise of the screw.

A sleeve bearing is insertable in the reinforcing rail or tube whichbearing is made of low-friction, plastic material and engages thethreads of the lead screw to offer lateral support in cooperation withthe reinforcing rail itself.

The projections on the nut portions have either partial threadsengageable with the threads of the lead screw or mount circular hubswhich are internally threaded to engage the lead screw.

The reinforcing rail may include at least one flat extending lengthwisewhich is engageable with mating flats on the nut portion to supplementthe anti-rotation function of the tongue and the slot in the rail toprevent the nut from rotating relative to the reinforcing rail. Theouter surface of the reinforcing rail may be coated with a low-frictionmaterial such as PTFE, Nylon or the like. The anti-backlash nut also maybe made of self-lubricating material such as acetyl and may include PTFEcarbon fiber additive or other lubricating additives for low frictionalengagement with the lead screw. It may be molded around or attached to aball bushing for added stiffness and lubricity. The sleeve bearinglocated within the lead screw may be made of Nylon, Delrin or likeplastic material.

Spring means such as compression springs or torsion springs force thenut halves or portions apart to urge the flanks of their threads intoforcible engagement with the flanks of the threads of the lead screw toprevent backlash while the nut translates laterally along thereinforcing rail in both directions.

The above and other features of the invention including various andnovel details of construction and combination of parts will now be moreparticularly described with reference to the accompanying drawings andpointed out in the claims. It will be understood that the particularreinforced lead screw embodying the invention is shown by way ofillustration only and not as a limitation of the invention. Theprinciples and features of this invention may be employed in varied andnumerous embodiments without departing from the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG 1. is a plan view of a reinforced lead screw embodying the presentinvention.

FIG. 2 is a partial sectional view taken along the lines II—II on FIG.1.

FIG. 3 is a sectional view through the reinforced guide rail and thenut-bearing taken along the lines III—III on FIG. 1 with the lead screwand reinforcing rail removed.

FIG. 4 is a view similar to FIG. 3 with the lead screw and thereinforcing rail removed and having one flat on the nut-bearing.

FIG. 5 is a schematic sectional view of part of the guide rail and thenut-bearing illustrating another embodiment of the flats.

FIG. 6 is a sectional view similar to FIG. 2 with the sleeve bearingremoved.

FIGS. 7 and 8 are views similar to FIGS. 3 and 4, respectively, of analternative construction of the nut-bearing.

FIG. 9 is a plan view of reinforced lead screw mounting an anti-backlashnut.

FIG. 10 is a view, partially in section, taken along the line XX of FIG.9.

FIG. 11 is a sectional view, taken along the line XI—XI of FIG. 9.

FIG. 12 is a sectional view, taken along the line XII—XII of FIG. 1, and

FIG. 13 is a sectional view, taken along the line XIII—XIII of FIG. 9.

FIG. 14 is a plan view, similar to FIG. 9, of another embodiment of theinvention showing a reinforced lead screw mounting anti-backlash nut ofa different construction.

FIG. 15 is a view, partially in section, taken along the line XV—XV ofFIG. 14.

FIG. 16 is a sectional view taken along the line XVI—XVI of FIG. 14.

FIG. 17 is a sectional view taken along the line XVII—XVII of FIG. 14.

FIG. 18 is a sectional view, taken along the line XVIII—XVIII of FIG.14.

DETAILED DESCRIPTION OF THE INVENTION

The reinforced lead screw is seen assembled in plan view in FIG. 1. Itcomprises a screw 2 rotatable about an axis α and having threads 4formed along its length. A projection 6 extends is formed on one end ofthe lead screw to connect it to a reversible driving motor (not shown)in order to rotate the lead screw alternatively in clockwise andcounterclockwise direction.

Surrounding lead screw 2 is an elongate reinforcing rail 8 or tube whichextends lengthwise of the screw. The reinforcing rail is hollow toaccommodate the lead screw and includes a slot 10 extending lengthwiseand parallel to the central axis α of the lead screw. The slot includesside walls 12 and 14 (FIGS. 3 and 5) which may constitute bearingsurfaces as will be explained in further detail hereinafter. The leadscrew is journaled within the reinforcing rail by bearings 16 and 18 forrotation relative to the reinforcing rail.

With reference to FIG. 2, there will be seen an elongate sleeve bearing22 extending lengthwise of the reinforcing rail. The bearing is made ofa low-friction plastic such as PTFE, Nylon or Delrin and has a circularinterior and engages the threads 24 of the lead screw. It functions as aradial support for the lead screw preventing the screw from whipping orvibrating as it is rotated.

The sleeve bearing 22 includes a slot 26 (FIG. 2) having sidewalls 28and 30 (FIG. 3) which, when assembled in the rail 8, are essentiallycontinuations of the sidewalls 12 and 14, respectively, of the slot 10in the reinforcing rail. The slots 10 and 26 are aligned with eachother.

In operation, the reinforcing guide rail 8 would be secured in a machinestructure by any convenient means (not shown) with a reversible drivingmotor attached to the lead screw 2.

A nut-bearing, generally indicated 40, which may be metal or,preferably, moldable plastic such as acetyl with carbon filter additivesis movable in bilateral direction along the reinforcing rail. Thenut-bearing may optionally have anti-backlash properties. Thenut-bearing includes a flange 42 which is illustrated as circular butmay be constructed in any convenient shape depending on the tool or loadto be attached. The nut-bearing includes a body portion 44 and which, aswill be seen in FIG. 4, is provided with an optional internal flat 46which is engageable with an optional mating flat 48 formed on the bottomof the reinforcing rail (FIG. 3).

The element 40 is called a nut-bearing because it functions as both. Itfunctions as a nut because, as will be seen hereinafter, it isinternally threaded and is moved by the lead screw. Secondly, it hasinternal bearing surfaces and slides on the rail.

As will be seen in FIG. 5, a plurality of flats 50 and 52 (hereinafterillustrated as only 2) may be formed in the nut-bearing which areengageable with mating flats 54 and 56 on the reinforcing rail 8.

As will be seen in FIG. 4, the nut-bearing 40 includes a projectingportion or tongue 51 extending radially inwardly. It has parallel sides53 which are engageable with the sides 12, 14, 28 and 39 of the alignedslots 10 and 26 formed in the guide rail 8 and the bearing 22,respectively. This engagement also assures that, in addition to thefunctioning of flats 46, 48, the nut will not rotate relative to theguide rail 8 when the lead screw is rotated.

The purpose of the optional flats 50, 42, 54, 56, be there one or aplurality, is to supplement the anti-rotational function of the tongue51 and the slots 10 and 26 in preventing the nut 40 from rotatingrelative to the reinforcing rail 8 when the lead screw 2 is rotated.This could be the case where the frictional engagement between thethreads of the lead screw and the nut is greater than the frictionalengagement between the nut and the exterior of the guide rail.

Arcuate threads 57 (FIG. 3) are located at the innermost end of theprojection 51. The threads are engageable with the threads 4 of the leadscrew 2 such that when the lead screw is rotated, either clockwise orcounterclockwise, the nut-bearing 40 will translate relative to thereinforcing rail 8.

As will be seen in FIGS. 7 and 8, an alternative construction of thenut-bearing 40 will be seen. A circular hub portion 58 is formed on thetorque or projection 51 and it has internal threads 60 (FIG. 8) whichencircle and engage the threads 24 of the lead screw 2. In FIG. 8, thesides 28 of the slot of the reinforcing rail 8 are shown separated fromthe sides 53 of the projection 51. This is optional construction. Theinternal threads 60 of the hub 58 are in mating engagement with thethreads of the lead screw.

Functionally, the FIG. 7 embodiment of the nut operates in the samemanner as the FIG. 3 embodiment.

FIG. 6 is a view similar to FIG. 2 with the sleeve bearing 22 removed.If a high degree of flexure of the lead screw were anticipated, thiswould not be done or, conversely, the reinforcing rail 8 would beconstructed with a smaller diameter such that it could engage the crestsof the threads formed on the lead screw. Furthermore, the longer thelead screw is, the more need there is for the reinforcing bearing tosupport the lead screw against whipping.

With the present invention, there is no need for lateral guide rails forthe nut, or if a carriage is attached, to ride on, since the reinforcingrail 8 is generally sufficient except in very large tool requirements.Since the breadth of the single rail construction is smaller, there isless chance for canting or skewing of the nut because its load can bemounted closer to the axis α. Circular loads can also be attached to thenut 44 surrounding the reinforced lead screw where there would be littleor no torque applied to the nut to cause it to bind on the guide rail 8.

Referring to FIG. 13, a two-part anti-backlash nut will be seen mountedon the reinforcing rail 8 for bilateral reciprocating motion imparted bythe rotatable lead screw 2. The anti-backlash nut comprises twoseparated nut halves 70 and 72. The nut half 70 has a cylindrical hubportion 76 and a larger diameter flange portion 74. The flange 18 mayalso be called a face plate. The nut half 70 through the flange 74 maybe bolted onto a load which, with the nut, is reciprocated back andforth by the reinforced lead screw. An open-wound, coiled compressionspring 78 is positioned around the outside of a tubular spacer 80. Oneend of the spring 78 abuts the nut half 72 and the other end abuts thehub portion 76 of the nut half 70. The central portion of the nut half72 is threaded to mesh with and rotatably engage the threads 4 of thelead screw 2, as will be described in greater detail hereinafter. Thenut halves are urged in opposite directions to ensure good anti-backlashcontact with the flanks of the threads 4 of the lead screw.

As hereinbefore described with reference to the nut-bearing 40, the nuthalf 72 includes a tongue 51 extending radially downwardly and inwardly,engaging the sidewalls 12 and 14 of the reinforcing rail 8. A circularhub 58 is formed on the tongue 51 and has internal threads 60 which arein mating engagement with the threads 4 of the lead screw 2. Whilelongitudinal movement is imparted to the nut half 72 by rotation of thelead screw 2, engagement of the tongue 51 with the sides 12 and 14 ofthe slot in the reinforcing rail 8 prevents the nut half from rotating.

The interior of the hub 76 of the nut half 70 is constructed in similarfashion as will be seen in FIGS. 11 and 13. A tongue 51′ extends intothe slot 10 if the reinforcing rail 8 and engages the sidewalls 12 and14. A similar circular hub 58′ has interior threads 60′ which likewiseengage the threads 4 of the rotatable lead screw.

As with the tongue 51, rotation of the nut halves 70, 72 is prevented bythe tongue 51 ′ engaging the walls 12 and 14 of the slot 10 of the leadscrew, while the compression spring 80 forces the nut halves 70, 72apart with their threads urged firmly into engagement with the flanks ofthe threads 4 of the lead screw to prevent backlash while theanti-backlash nut translates bilaterally along the reinforcing rail 8.

Referring next to FIGS. 14-18, another embodiment of an anti-backlashnut mounted on the reinforcing rail 8 will be illustrated. A two part,split, anti-backlash nut 81 is mounted for bilateral reciprocatingmotion on the rail 8 imparted by the rotatable lead screw 2. The nut 81comprises essentially two separated compound nut halves 82 and 83. Thenut half 83 has an internally threaded cylindrical hub portion 84 and anattached, larger diameter flange portion 86. As with the previouslydescribed FIG. 9 embodiment, the flange 86 may also be called a faceplate. It may be bolted to a load which, with the combined nut 81 isreciprocated back and forth by the reinforced lead screw 2. Anopen-wound, coiled torsion spring 88 with tangs 90 and 92 at its endssurrounds a tubular spacer 94, one end of which abuts the nut half 83.The other half is threaded at 96, and terminates in a face 98 whichengages an anti-friction member 98 in the form of a high-frictionelastomeric washer 100. The washer has a circular concentric opening 99which fits around the reinforcing rail 8 such that the elastomericwasher 100 may be slid over the rail during assembly. The high-frictionelastomeric washer 100, when assembled, abuts the face 102 of nut half82.

Each of the nut halves 82 and 83 is threaded onto the lead screw 2, asshown in FIG. 15 and will be described in greater detail hereinafter.The tang 92 of the torsion spring 88 is inserted into a recess in thecollar portion 104 threaded on the spacer 94. A corresponding collar 106is threaded onto the threads 96 of the spacer 94. The torsion spring 88is given a twist with the other tang 90 inserted in a bore preformed inthe collar 106. The high-friction elastomeric washer 100, when assembledas shown in FIG. 15, is located between the face 98 of the collar 106and a face 102 on a cylindrical, internally threaded, portion 85 of thenut half 82. With the parts assembled as shown in FIG. 15, the torsionspring 88 continuously urges the nut halves apart to take up any slackbetween the flanks of the threads 4 on the lead screw and the internallythreaded portions of the nut halves 82 and 83. Hence there will be nolost movement because the thread flanks are at all times in intimatecontact regardless of the direction of movement.

The torsion spring's ability to rotate one nut half with respect to theother maintains intimate contact between the nut half collar 106 withthe elastomeric friction washer 100 and the thread flanks of the nuthalves with the engaged thread flanks of the lead screw 2. Thehigh-friction elastomeric washer 100 limits the relative rotationalmovement between the collar 106 and the back portion 85 of the nuthalves when they are in the closed position to prevent backlash fromreoccurring.

The invention claimed is:
 1. A reinforced lead screw assemblycomprising: a threaded lead screw rotatable about a central axis; ahollow, elongate reinforcing rail extending lengthwise of andsurrounding the lead screw; a slot in the reinforcing rail extendinglengthwise of the central axis; a nut-bearing moveable along thereinforcing rail; a portion of the nut extending through the slot;internal threads on the extending portion of the nut-bearing engageablewith the threads on the lead screw for moving the nut-bearing inreciprocating motion lengthwise of the rail when the lead screw isrotated wherein the nut-bearing threads are formed on the portion of thenut extending through the slot in an arc.
 2. A reinforced lead screwassembly according to claim 1, having at least one flat extendinglengthwise of the reinforcing rail and a mating flat in the nut-bearingto prevent the nut from rotating relative to the reinforcing rail.
 3. Areinforced lead screw assembly according to claim 1, having a pluralityof flats extending lengthwise of the reinforcing rail and mating flatsin the nut-bearing to prevent the nut from rotating relative to thereinforcing rail.
 4. A reinforced lead screw assembly comprising: athreaded lead screw rotatable about a central axis; a hollow elongatereinforcing rail extending lengthwise of and surrounding the lead screw;a slot in the reinforcing rail extending lengthwise of the central axis;an elongate bearing within the reinforcing rail having a slot alignedwith the slot in the reinforcing rail; a nut-bearing moveable along thereinforcing rail; a portion of the nut extending through the slot;internal threads in the extending portion of the nut-bearing engageablewith the threads on the lead screw for moving the nut in reciprocatingmotion lengthwise of the rail when the lead screw is rotated wherein theportion of the nut-bearing extending through the slots mounts a circularhub which has internal threads engageable with the threads of the leadscrew.
 5. A reinforced lead screw assembly comprising: a threaded leadscrew rotatable about a central axis; a hollow, elongate reinforcingrail extending lengthwise of and surrounding the lead screw; a slot inthe reinforcing rail extending lengthwise of the central axis; anut-bearing moveable along the reinforcing rail; a portion of the nutextending through the slot; wherein the portion of the nut-bearingextending through the slot mounts a circular hub which has internalthreads engageable with the threads of the lead screw for moving thenut-bearing in reciprocating motion lengthwise of the rail when the leadscrew is rotated.
 6. A reinforced lead screw assembly comprising: athreaded lead screw rotatable about a central axis; a hollow elongatereinforcing rail extending lengthwise of and surrounding the lead screw;a slot in the reinforcing rail extending lengthwise of the central axis;an elongate bearing within the reinforcing rail having a slot alignedwith the slot in the reinforcing rail; a nut-bearing moveable along thereinforcing rail; a portion of the nut extending through the slot;internal threads in the extending portion of the nut-bearing engageablewith the threads on the lead screw for moving the nut in reciprocatingmotion lengthwise of the rail when the lead screw is rotated wherein thenut-bearing threads are formed on the portion of the nut extendingthrough the slots in an arc.
 7. A reinforced lead screw assemblyaccording to claim 6, comprising at least one flat extending lengthwiseof the reinforcing rail and at least one mating flat in the nut-bearingto prevent the nut from rotating relative to the reinforcing rail.
 8. Areinforced lead screw assembly according to claim 6, having a pluralityof flats extending lengthwise of the reinforcing rail and mating flatsin the nut-bearing to prevent the nut from rotating relative to thereinforcing rail.
 9. A reinforced lead screw assembly according to claim6, wherein the sleeve-bearing has an arcuate interior engageable withthe threads of the lead screw.
 10. A reinforced lead screw assemblyaccording to claim 6, wherein the sleeve-bearing is made of a lowfriction plastic material.
 11. A reinforced lead screw assemblycomprising: a threaded lead screw rotatable about a central axis; ahollow elongate reinforcing rail extending lengthwise of and surroundingthe lead screw; a slot in the reinforcing rail extending lengthwise ofthe central axis; bearing surfaces forming the sides of the slot; anelongate sleeve-bearing within the reinforcing rail having a slotaligned with the slot in the reinforcing rail; a nut-bearing moveablealong the reinforcing rail; a projection on the nut extending throughthe aligned slots and engageable with the bearing surfaces; threads onthe nut-bearing engageable with the threads on the lead screw for movingthe nut-bearing in reciprocating motion lengthwise of the rail when thelead screw is rotated wherein the projection mounts a circular hub whichhas internal threads engageable with the threads of the lead screw. 12.An anti-backlash nut assembly on a reinforced lead screw comprising: athreaded lead screw rotatable about a central axis; a hollow, elongatereinforcing rail extending lengthwise of and surrounding the lead screw;a slot in the reinforcing rail extending lengthwise of the central axis;an anti-backlash nut assembly having a nut moveable along thereinforcing rail; the anti-backlash nut comprising two separate nutportions; each nut portion having a tongue extending radially throughthe slot and having threads engageable with the threads of the leadscrew for moving both nut portions in reciprocating motion lengthwise ofthe rail when the screw is rotated; a compression spring for forcing thenut portions apart to urge the flanges of their threads into forcibleengagement with the threads of the lead screw to prevent backlash whilethe anti-backlash nut translates laterally along the reinforcing rail.13. A reinforced lead screw assembly comprising: a threaded lead screwrotatable about a central axis; a hollow elongate reinforcing railextending lengthwise of and surrounding the lead screw; a slot in thereinforcing rail extending lengthwise of the central axis; bearingsurfaces forming the sides of the slot; an elongate sleeve-bearingwithin the reinforcing rail having a slot aligned with the slot in thereinforcing rail; a nut-bearing moveable along the reinforcing rail; aprojection on the nut extending through the aligned slots and engageablewith the bearing surfaces; threads on the nut-bearing engageable withthe threads on the lead screw for moving the nut-bearing inreciprocating motion lengthwise of the rail when the lead screw isrotated wherein the nut-bearing threads are formed on the projection inan arc.
 14. A reinforced lead screw assembly according to claim 13,comprising at least one flat extending lengthwise of the reinforcingrail and at least one mating flat in the nut-bearing to prevent thenut-bearing from rotating relative to the reinforcing rail.
 15. Areinforced lead screw assembly according to claim 13, having a pluralityof flats extending lengthwise of the reinforcing rail and mating flatsin the nut-bearing to prevent the nut-bearing from rotating relative tothe reinforcing rail.
 16. An anti-backlash nut assembly on a reinforcedlead screw comprising: a threaded lead screw rotatable about a centralaxis; a hollow, elongate reinforcing rail extending lengthwise of andsurrounding the lead screw; a slot in the reinforcing rail extendinglengthwise of the central axis; an anti-backlash nut assembly having anut moveable along the reinforcing rail; the anti-backlash nutcomprising two separate nut portions; each nut portion having a tongueextending radially through the slot and having threads engageable withthe threads of the lead screw for moving both nut portions inreciprocating motion lengthwise of the rail when the screw is rotated; acoiled torsion spring for forcing the nut portions apart to urge theflanks of their threads into forcible engagement with the threads of thelead screw to prevent backlash while the anti-backlash nut translateslaterally along the reinforcing rail.
 17. An anti-backlash nut assemblyon a reinforced lead screw according to claim 16 wherein the torsionspring has tangs on each end, engageable, one each in the separate nutportions.
 18. An anti-backlash nut assembly on a reinforced lead screwcomprising: a threaded lead screw rotatable about a central axis; ahollow, elongate reinforcing rail extending lengthwise of andsurrounding the lead screw; a slot in the reinforcing rail extendinglengthwise of the central axis; an anti-backlash nut assembly having anut moveable along the reinforcing rail; the anti-backlash nutcomprising two separate nut portions; each nut portion having a tongueextending radially through the slot and having threads engageable withthe threads of the lead screw for moving both nut portions inreciprocating motion lengthwise of the rail when the screw is rotated;and a coiled torsion spring for forcing the nut portions apart to urgethe flanks of their threads into forcible engagement with the threads ofthe lead screw to prevent backlash while the anti-backlash nuttranslates laterally along the reinforcing rail.
 19. An anti-backlashnut assembly on a reinforced lead screw according to claim 18, whereinthe means for forcing the nut halves apart is a coiled torsion springsurrounding the reinforcing rail and wherein the torsion spring hastangs on each end, engageable, one each in the separate nut portions.20. An anti-backlash nut assembly on a reinforced lead screw accordingto claim 18, wherein a tubular spacer is located between the compressionspring and the reinforcing rail.
 21. An anti-backlash nut assembly on areinforced lead screw according to claim 18, wherein hubs are formed oneach of the tongues and the threads which engage the threads of the leadscrew are formed in the hubs.
 22. An anti-backlash nut assembly on areinforced lead screw according to claim 18, wherein the tongues areengageable with the sides of the slot to prevent rotation of the nuthalves as they translate lengthwise of the lead screw.
 23. Ananti-backlash nut assembly on a reinforced lead screw according to claim18, wherein the tongues are engageable with the sides of the slot toprevent rotation of the nut halves as they translate lengthwise of thelead screw.
 24. An anti-backlash nut assembly on a reinforced lead screwaccording to claim 18, wherein a tubular spacer is located between thetorsion spring and the reinforcing rail.
 25. An anti-backlash nutassembly on a reinforced lead screw according to claim 18, wherein hubsare formed on each of the tongues and the threads which engage thethreads of the lead screw are formed in the hubs.